1. Field of the Invention
The present invention relates to Doppler spread estimation in a wireless communication system. More particularly, the present invention relates to an apparatus and a method for estimating a Doppler frequency in a wireless mobile communication system.
2. Description of the Related Art
In a wireless mobile communication system, a signal is scattered, reflected, and attenuated due to obstacles, such as a building, a vehicle, or geographic features, and propagates as a multipath signal having multi-delay. In the multipath propagation environment, when a receiver, for example, a terminal moves, the phase of the multipath signal components arriving at the terminal varies and thus the received signal is subject to Doppler spread. The relationship between a velocity of the terminal and a maximum value of the Doppler spread is expressed as Equation (1).
                    v        =                              c            ·                          f              m                                            f            c                                              (        1        )            
In Equation (1), v denotes the velocity of the terminal, c denotes the velocity of light, fm denotes a maximum Doppler frequency, and fc denotes a subcarrier frequency.
To accurately detect the channel change according to the movement of the terminal and to adaptively set the bandwidth of filters used in the receiver, the wireless mobile communication system estimates the Doppler frequency according to the velocity of the terminal using one of a variety of methods. For example, the related art provides methods for estimating the Doppler frequency using Zero-Crossing-Rate (ZCR), Level-Crossing-Rate (LCR), Auto-Correlation-Function (ACF), and COVariance (COV). Herein, the ZCR method and the LCR method estimate the maximum Doppler frequency using the zero crossing or the level crossing of a random fading signal, the ACF method estimates the maximum Doppler frequency using an auto-correlation function of the receive signal, and the COV method estimates the maximum Doppler frequency using the covariance of the received signal power. However, while such methods for estimating the maximum Doppler frequency offer an adequate estimation performance when a Signal to Noise Ratio (SNR) is high, the estimation performance is degraded in the low-SNR environment where the noise power is equal to the signal.
A Doppler estimation method using Power Spectral Density (PSD) is also provided. The PSD method directly calculates the power spectrum of the channel by applying a Fast Fourier Transform (FFT) to the channel value of a continuous pilot channel transmitted in the time domain. However, this method requires the additional FFT in a Code Division Multiple Access (CDMA) system, which increases hardware complexity and based on the additional computation.
By contrast, as an Orthogonal Frequency Division Multiplexing (OFDM) system basically performs the FFT, there is no need to add the FFT means to obtain the power spectrum. Accordingly, the method using the power spectrum determined using the FFT is suitable for the OFDM system. Still, since the pilot is inserted to some subcarriers of the frequency domain in the OFDM system, it is not easy to directly use the pilot in the time domain.
Therefore, a need exists for an apparatus and a method for estimating a Doppler frequency in a mobile communication system.